JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MACHINE DRAWING AND COMPUTER AIDED GRAPHICS Previous Question Paper (December - 2018 - R15)

Thread profiles are represented in machine drawings using simplified conventions to indicate their specific geometry and application. The representation typically involves showing the profile shape in a sectional view or a detailed view.

Square Threads: These are represented by a series of rectangles with a pitch equal to twice the thread thickness. The depth of the thread is equal to 0.5 times the pitch. In a sectional view, the threads appear as a succession of squares, emphasizing the flat, perpendicular flanks used for power transmission.

ACME Threads: These are represented by a trapezoidal profile with an included angle of 29 degrees. The drawing shows the threads with a flatter crest and root compared to standard V-threads. The representation clearly indicates the trapezoidal shape, which is designed for engagement with split nuts and power transmission applications.

Buttress Threads: These are represented by a non-symmetrical profile where one flank is perpendicular to the axis and the other is inclined at 45 degrees. The drawing must clearly distinguish the load-bearing side (the perpendicular side) from the non-load-bearing side. This specific profile is used for applications requiring high axial force in one direction only, such as in jacks or vice screws.

A Gib and Cotter joint is used to connect two rods of rectangular cross-section, allowing for slight adjustments in length. The joint consists of a rod end, a sleeve (or strap), a cotter, and a gib.

The proportions for the design are typically based on the diameter of the rod (d). For a rod of diameter d, the standard proportions are:

• Width of cotter at the center = 1.3d
• Thickness of cotter = 0.3d
• Taper of cotter = 1 in 24
• Width of gib = 0.5d
• Thickness of gib = 0.3d
• Length of the sleeve/strap = 4d to 5d

The assembly drawing requires two views: a longitudinal section and a cross-section. The longitudinal section shows the rods placed inside the sleeve, with the cotter and gib inserted through the common slot. The gib is placed on the side of the slot to provide a flat bearing surface for the cotter, preventing the rod ends from splaying. The cross-section is taken through the center of the cotter to show the arrangement of the rod, gib, and cotter within the sleeve.